1. Field of the Invention
The present invention relates to an apparatus and a method for transmitting and receiving control information in a Multiple Input Multiple Output (MIMO) system. More particularly, the present invention relates to an apparatus and a method for transmitting and receiving Single User (SU)-MIMO control information and Multiple User (MU)-MIMO control information in the MIMO system.
2. Description of the Related Art
In response to increasing demand for high-speed and high-quality data transmission, Multiple Input Multiple Output (MIMO) technology using a plurality of transmit antennas and receive antennas is drawing much attention. The MIMO technology carries out communication over a plurality of channels with the plurality of the antennas and thus greatly enhances a channel capacity, compared to a single antenna. For example, when both of a transmitter and a receiver include M-ary transmit antennas and receive antennas, channels between the antennas are independent of each other, and a bandwidth and a whole transmit power are fixed, an average channel capacity increases M times, compared to the single antenna.
The MIMO technology may be divided into Single User (SU)-MIMO and Multiple User (MU)-MIMO. The SU-MIMO allows a pair of the transmitter and the receiver to perform one-to-one communication by occupying all of the channels with the multiple antennas, and the MU-MIMO performs communication between the transmitter and the receiver in one-to-many relation by splitting the channels with the multiple channels.
Meanwhile, a Long Term Evolution (LTE)-advanced system which is an International Mobile Telecommunication (IMT)-advanced candidate, utilizes multi-user scheduling gain in a space domain at maximum, and is considering a technique for extending 44 downlink SU-MIMO of the existing LTE Rel-8 to 88 downlink SU-MIMO, and a downlink MU-MIMO technique as its core techniques to attain higher maximum spectral efficiency and average cell throughput than the downlink MIMO. While the LTE Rcl-8 system also adopts the MU-MIMO, it applies the most simplified MU-MIMO based on conventional Spatial Division Multiple Access (SDMA). Thus, a base station provides a terminal with no information relating to the same channel interference from other users scheduled together with the corresponding terminal. The only difference between the MU-MIMO and the SU-MIMO according to the LTE Rel-8 standard is whether information relating to the transmit power sharing is contained in a corresponding Physical Downlink Control CHannel (PDCCH).
To properly use the spatial diversity gain of the MU-MIMO, the same channel interference between the transmitter and the receiver should be adequately handled, so as to reduce performance degradation caused by the same channel interference. For doing so, the LTE-advanced system is considering not only control information for the SU-MIMO but also additional control information for the MU-MIMO, such as the number of co-scheduled User Equipments (UEs), actual transmission rank and port of all the co-scheduled UEs, exact resource allocation of all the co-scheduled UEs, and Modulation and Coding Scheme (MCS) level. Accordingly, details of the control information transmission for the MU-MIMO are of great concern to the current LTE-advanced MIMO.
The control information transmission for the MU-MIMO currently under consideration includes largely two methods.
The first method supports the SU-MIMO and the MU-MIMO in different transmission modes. When additional control information for distinguishing the SU-MIMO mode and the MU-MIMO mode is transmitted, overhead of the additional transmission over a control channel increases in terms of the base station. When the first method considers higher layer signaling to distinguish the SU-MIMO mode and the MU-MIMO mode, the transmission mode is semi-statically configured to thus deteriorate scheduling flexibility.
The second method supports the SU-MIMO and the MU-MIMO in the same transmission mode with different Downlink Control Information (DCI) formats. Since the terminal has to monitor the two DCI formats of the different lengths in the control channel at the same time, complexity in the decoding increases.
As discussed above, the current MIMO control information transmission methods are inefficient in view of the MIMO support. Thus, what is needed is a more rational control information transmission method for supporting the SU-MIMO and the MU-MIMO to attain a maximum gain through the efficient MIMO support.